Accumulation of lead in tree trunk bark pockets as pollution time capsules

The concentration of lead in bark pockets of tree trunks was investigated for its potential use as an archival indicator of pollution. The bark pockets investigated were those of a conifer, Cryptomeria japonica, formed around 1760–1780 (235–255 years ago) at Nikko about 100 km north of Tokyo, and around 1786–1809 (186–209 years ago) on Yakushima Island, located in a remote southern region of Japan. The lead concentrations in these C. japonica bark pockets, representing the total lead accumulation during a period of about 20 years at each site, were 0.1 μg Pb g−1 at Nikko and 0.22 μg Pb g−1 at Yakushima. In contrast, the lead concentration in the outer bark of C. japonica at the present time is about 150 μg Pb g−1 at Nikko (1990) and 1.4 μg Pb g−1 at Yakushima (1992). The use of leaded gasoline, the main source of lead in the atmosphere, was initiated in Japan in 1949 and reached a maximum during 1960–1965. As the production of leaded gasoline was stopped in 1987, the lead concentration in the outer bark represents the total for a period of about 40 years. Therefore, these results suggest an increase in lead pollution of about three orders of magnitude at Nikko, which is relatively close to Tokyo, and one order of magnitude at Yakushima, which is relatively remote.